Secondary batteries such as nickel cadmium batteries, nickel hydrogen batteries, and lithium ion secondary batteries are used in electric devices and electronic devices such as mobile phones and laptop computers. Lithium ion secondary batteries in particular are often used in mobile phones and other mobile devices due to their high weight energy density and suitability for small and lightweight applications. In recent times, lithium ion secondary batteries have been used in an increasingly wide range of fields, as seen in their use in power tools, electric vehicles, hybrid cars, and small satellites.
In these fields, there has been a demand for reductions in size, weight, and cost as well as for improvements in weight energy density and safety. A material used for armoring of a battery such as a lithium ion secondary battery is typically a metal foil which has been coated with resin in a laminated manner on one or both sides thereof. Such a material is heat sealed so as to produce a battery container. Along with the above-described increasing demands, there has also been a need for a battery container to be shaped so as to allow battery components to efficiently fill the battery container without dead space. As such, there is a demand for metal foil as a battery armoring material, since metal foil exhibits superior processability in press working such as bulging. Aluminum foil and aluminum alloy foil are often used due to the ease which they can be made thin and press worked. For example, Patent Literature 1 discloses a battery armoring material which is obtained by coating an aluminum alloy foil with polypropylene (PP) in a laminated manner, the foil containing Fe in an amount of not less than 0.6%. Patent Literature 1 discloses that the material is useful for size and weight reduction.
Unfortunately, further reductions in the thickness of aluminum foil will reduce its breaking strength and therefore make the material more likely to break during press working. There is also the risk that an external force such as a vibration, an impact, or a piercing force acting on the battery armoring material may cause the material to become deformed or damaged and that electrolyte solution will leak out from the battery as a result.
Furthermore, lithium ion secondary batteries currently use an electrolyte solution obtained by dissolving LiPF6 into a mixed solution containing ethylene carbonate and dialkyl carbonic ester. Such an electrolyte solution is known to react with water to produce hydrogen fluoride. Because hydrogen fluoride is highly corrosive, a leak of electrolyte solution is likely to significantly damage a device to which a lithium ion battery is mounted. As such, from the standpoint of safety, there is a demand for a metal foil having a high breaking strength for use as a battery armoring material.
There have therefore been recent developments in techniques involving stainless steel foil as a battery armoring material which can fulfill the above-described property requirements. Stainless steel foil has high strength and is typically several times stronger than aluminum foil in terms of tensile strength and the like. Compared to conventional materials for a battery armoring, stainless steel foil can be made to be thinner and is safer.
An important parameter of a battery armoring material is adhesiveness between the metal foil and the resin coated thereon in a laminated manner on a side that comes in contact with electrolyte solution. In a case where there is poor adhesiveness between the resin and the metal foil, electrolyte solution which permeates into the resin over time will cause the resin to peel from the metal foil once the electrolyte solution reaches the surface of the metal foil. This creates the risk of the electrolyte solution leaking. Patent Literatures 2 and 3 disclose methods of improving the adhesiveness of the stainless steel foil itself with respect to resin. These methods involve subjecting the stainless steel foil to heat treatment in a reducing atmosphere so as to provide, to the stainless steel foil, an oxide film having superior adhesiveness.
Patent Literature 2 discloses subjecting a stainless steel sheet, having a random abrasion pattern or a hairline abrasion pattern, to bright annealing at a temperature of not less than 800° C. so as to provide an oxide film in which Si content is increased to not less than 50 mol percent. This improves adhesiveness between the stainless steel and resin such as epoxy-based and polyester-based resin.
Patent Literature 3 discloses annealing a stainless steel foil which has been given an arithmetic mean roughness Ra of not less than 0.1 μm by use of hairline abrasion or a reduction roll having a rough surface. The stainless steel foil is annealed at a temperature in a range of 600° C. to 800° C. so as to obtain an oxide film in which (i) oxygen content is within a range of 20 mol percent to 60 mol percent and (ii) Cr content is higher than Fe content. This improves adhesiveness between the stainless steel foil and polyolefin-based resin.